1. Field of the Invention
The present invention relates to a semi-conductor device, and more particularly, to a compound semiconductor device with a heterojunction.
2. Description of the Related Art
Recent developments in crystal growth techniques have made it possible to obtain a superior semiconductor heterojunction, and thus an abrupt change and a flatness at a heterojunction interface between a compound semiconductor layer and another compound semiconductor layer can be controlled at a monoatomic layer level. For example, a HEMT (High Electron Mobility Transistor) using a single heterojunction of III-V group compound layers has been realized.
In addition to the HEMT, a HET (Hot-Electron Transistor) and a RHET (Resonant Tunneling Hot Transistor) and the like, which use a carrier movement in a direction perpendicular to the heterojunction interface, have been studied and developed, and further, other semiconductor devices utilizing a quantum-mechanical effect are now being studied and developed: These devices try to realize new functions or higher speed operations.
A basic semiconductor device element using a conventional resonant tunneling effect is now explained, using a conventional resonant tunnel diode shown in FIGS. 1a and 1b and FIG. 2 as an example.
As shown in FIG. 1a, using an MBE (Molecular-Beam Epitaxy) method, an AlGaAs thin layer 102, a GaAs thin layer 104, an AlGaAs thin layer 106 and a GaAs layer 108 are epitaxially deposited (grown) on a GaAs layer 100 in sequence, to form heterojunction interfaces between the adjoining layers, and electrodes 110 and 112 are formed on the GaAs layers 100 and 108, respectively. A predetermined bias voltage is applied across the electrodes 110 and 112, and an ammeter A is connected in series between the power source and the electrode 112.
Since the energy of the conduction band edge of AlGaAs is higher than that of the conduction band edge of GaAs, as shown in FIG. 1b, the AlGaAs thin layers 102 and 106 form potential barriers, but where the thickness of the layers 102, 104 and 106 is very thin, the potential barriers of the AlGaAs thin layers become tunnel barriers, and the GaAs thin layer 104 sandwiched by the tunnel barriers forms a quantum well. In the quantum well, specific electron energies (wavelengths) are multi-reflected to provide a remarkable interference effect, with the result that the transmission coefficient (probability) of the tunnel barriers is increased and a resonant tunneling effect is demonstrated.
The resonant tunnel diode has the current-voltage (I-V) characteristics shown in FIG. 2 and causes an NDR (Negative Differential Resistance), which NDR is applied to a detection of submillimeter-waves (T. C. L. G. Sollner, et al.: Appl. Phys. Lett., 43 (6), 15 September (1983) 588-590), a Transistor (N. Yokoyama, et al.: Jan. J. Appl. Phys., 24 (11) November (1985) L853-L854) or the like. When a device utilizes the NDR, preferably an inclination of a broken line H in FIG. 2 is steeper and/or a peak-to-valley (PV) ratio is higher.
In studies of semiconductor devices having heterojunctions, a formation of minibands due to a limitation in one direction of the electron movement has been applied to various devices, in line with the semiconductor superlattice proposed by Esaki et al. Further, in addition to one-dimensional superlattice consisting of alternating ultra-thin layers, attempts have been made to use a two-dimensional superlattice and a three-dimensional superlattice, which further limit the degrees of freedom of the electron, as a channel of an FET (Field Effect Transistor) or as an active layer of a laser.
Furthermore, proposals have been made with regard to devices using a quantum-mechanical effect, e.g., an AB (Aharonov-Bohm) effect device. The AB effect is such that a quantum-mechanical electron phase is influenced by an electromagnetic potential. For example, an AB effect device has a conductor ring provided with two leads extended diametrically and connected to a source and a drain, respectively. In this device, an electron wave from the source is divided at the inlet of the ring, flows through the two halves (passes) of the ring, respectively, merge again at the outlet of the ring, and then flow through the other lead to the drain. A phase difference between the electron waves flowing in the two (different) passes of the ring depends on a magnetic flux passed through the inside of the ring or on a scalar potential applied to one of the passes of the ring. Therefore, a suitable control means for regulating the magnetic flux within the ring or the scalar potential on one of the two passes is provided, to a generate an electron wave interference effect due to the phase difference, by which a conductance between the source and drain can be controlled.
Accordingly, the AB effect device can be used as a switching element or as an amplifying element, and can operate at a very high speed, since only a very small variation of an input signal is required for controlling the device, compared with a conventional transistor. It is expected that the AB effect device can be applied to an ultra-high speed integrated circuit.
Nevertheless, a semiconductor device using the conventional resonant tunneling effect has a layer-piled structure (consisting of the AlGaAs thin layer 102, GaAs thin layer 104, and AlGaAs thin layer 106), and thus the AlGaAs thin layers 102 and 106 form two-dimensional potential barriers and the conduction electrons move three-dimensionally. In this case, among the three direction momentums of the conduction electrons, only the momentum component in a "x" direction perpendicular to the potential barrier is related to a tunneling probability.
Namely, where an energy difference (E=eV) is provided on both sides of a potential barrier, as shown in FIG. 3, when the conduction electrons have a Fermi level of ".mu.", the maximum value K.sub.MAX and the minimum values K.sub.MIN of a wave vector of conduction electrons tunneling through the barrier at the absolute zero are given by: EQU K.sub.MAX =[2m .mu./h .sup.2 ].sup.1/2 EQU K.sub.MIN =[2 m(.mu.-eV)/h .sup.2 ].sup.1/2.
Therefore, a portion of the conduction carriers having the "x" direction wave vector contributing to the tunneling current is limited to S(K.sub.x) in the wave number space, as shown in FIGS. 4a and 4b (cf. R. B. Eloyd and D. G. Walmsley, J. Phys. C11, (1978) 4601). A bias varies to vary the radius of the inside sphere. The tunneling current J flowing through the potential barrier at absolute zero is given by: EQU J=dK.sub.x .multidot.2e.multidot.V.sub.x (K.sub.x).multidot.S(K.sub.x).multidot.[S/4.pi..sup.2 ]X T(K.sub.x)
where T(K.sub.x) is a tunneling probability, the integration is performed from K.sub.x =0 to K.sub.x =K.sub.MAX, V.sub.x is the group velocity of the conduction electrons with the wave vector K.sub.x, and S is the area of the barrier. The bias eV vary not only T(K.sub.x) but also S(K.sub.x). In this formula, a multiplication by S(K.sub.x) and an integration contribute an averaging effect, whereby the NDR in the I-V characteristic is lowered, although the tunneling probability has a sharp peak at a certain value of K.sub.x. Namely, the resonant tunnel effect of the semiconductor element using this effect is weakened and remarkable NDR is not obtained. From another standpoint, an reason for improving the NDR characteristics including a PV (peak to valley) ratio is now explained in connection with a number of states of electron. If the tunneling probability is ignored, a number N of electron states which can tunnel an barrier in view of energy, is calculated by dividing a difference between a volume of n (n=3 or 2)-dimensional sphere having a radius of k.sub.MAX and that of n-dimensional sphere having a radius of k.sub.MIN by (2.pi./L).sup.n. At n=3, i.e., under three dimensions in a conventional resonant tunnel device, ##EQU1## therefore ##EQU2## Where V.sub.0 is an n-dimensional volume of a semiconductor device. At n=2, i.e., under twodimensions in the resonant tunnel devise according to the present invention, ##EQU3## Therefore, the above-mentioned relationships can be illustrated, as shown in a graph of FIG. 5. As can be seen from FIG. 5, when the bias increases, the number N of tunnelable states in the three-dimension case increases more than that in the two-dimension case. In other words, the dimension number n is decreased from 3 to 2 to make dependency of the number N of tunnelable states on the bias smaller, with the result the PV ratio or the like are enlarged (improved).
It is still thought impossible to realize a semiconductor device having an artificial modulation of an electron band structure and an operation characteristic not attained by a conventional device, by using the two-dimensional and three-dimensional superlattice limiting the movements of the electron in not only one certain direction but also other directions. Accordingly, intensive studies into the realization of such a device must be made. Furthermore, to generate the quantum-mechanical interference effect of the AB effect device, it is necessary to form an extreme pass (conductor line) to thereby one-dimensionally restrict the electron movement. It is, however, difficult to attain a very narrow width of the pass by a conventional mesa-etching method, and thus a practical AB effect device has not been realized, and this is also a matter for intensive study.